U.S. patent number 8,368,995 [Application Number 13/076,205] was granted by the patent office on 2013-02-05 for method and system for hybrid integration of an opto-electronic integrated circuit.
This patent grant is currently assigned to Skorpios Technologies, Inc.. The grantee listed for this patent is John Dallesasse, William Kozlovsky, Stephen B. Krasulick. Invention is credited to John Dallesasse, William Kozlovsky, Stephen B. Krasulick.
United States Patent |
8,368,995 |
Dallesasse , et al. |
February 5, 2013 |
Method and system for hybrid integration of an opto-electronic
integrated circuit
Abstract
An opto-electronic integrated circuit (OEIC) includes an SOI
substrate, a set of composite optical transmitters, a set of
composite optical receivers, and control electronics disposed in
the substrate and electrically coupled to the set of composite
optical transmitters and receivers. Each of the composite optical
transmitters includes a gain medium including a compound
semiconductor material and an optical modulator. Each of the
composite optical receivers includes a waveguide disposed in the
SOI substrate, an optical detector bonded to the SOI substrate, and
a bonding region disposed between the SOI substrate and the optical
detector. The bonding region includes a metal-assisted bond at a
first portion of the bonding region and a direct
semiconductor-semiconductor bond at a second portion of the bonding
region. The OEIC also includes control electronics disposed in the
SOI substrate and electrically coupled to the set of composite
optical transmitters and the set of composite optical
receivers.
Inventors: |
Dallesasse; John (Geneva,
IL), Krasulick; Stephen B. (Albuquerque, NM), Kozlovsky;
William (Sunnyvale, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Dallesasse; John
Krasulick; Stephen B.
Kozlovsky; William |
Geneva
Albuquerque
Sunnyvale |
IL
NM
CA |
US
US
US |
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Assignee: |
Skorpios Technologies, Inc.
(Albuquerque, NM)
|
Family
ID: |
44858060 |
Appl.
No.: |
13/076,205 |
Filed: |
March 30, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110267676 A1 |
Nov 3, 2011 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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12903025 |
Oct 12, 2010 |
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61251143 |
Oct 13, 2009 |
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Current U.S.
Class: |
359/279; 398/141;
385/15 |
Current CPC
Class: |
H01S
5/141 (20130101); H01S 5/021 (20130101); H01S
5/1032 (20130101); H01S 5/0237 (20210101); H01S
5/06246 (20130101); H01S 5/02325 (20210101); H01S
5/0261 (20130101) |
Current International
Class: |
G02F
1/01 (20060101); G02B 6/26 (20060101) |
Field of
Search: |
;359/279 ;372/50.1,50.11
;385/15 ;398/140,141 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 2011/046898 |
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Apr 2011 |
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WO |
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Other References
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.
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Primary Examiner: Spector; David N
Attorney, Agent or Firm: Kilpatrick Townsend & Stockton
LLP
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This present application is a continuation-in-part of U.S. patent
application Ser. No. 12/903,025, filed on Oct. 12, 2010, which
claims priority to U.S. Provisional Patent Application No.
61/251,143, filed on Oct. 13, 2009, the disclosures of which are
hereby incorporated by reference in their entirety for all
purposes. U.S. patent application Ser. No. 12/903,025 was filed
concurrently with 12/902,621, the disclosure of which is hereby
incorporated by reference in its entirety for all purposes.
Claims
What is claimed is:
1. An opto-electronic integrated circuit comprising: a substrate
comprising a silicon material; a set of optical transmitters, each
comprising: a gain medium coupled to the substrate, wherein the
gain medium includes a compound semiconductor material; an optical
modulator optically coupled to the gain medium; a waveguide
disposed in the substrate and optically coupled to the gain medium;
a first wavelength selective element characterized by a first
reflectance spectrum and disposed in the substrate; a second
wavelength selective element characterized by a second reflectance
spectrum and disposed in the substrate; an optical coupler disposed
in the substrate and joining the first wavelength selective
element, the second wavelength selective element, and the
waveguide; and an output mirror; a set of optical receivers, each
comprising: a waveguide disposed in the silicon material; an
optical detector bonded to the silicon material; and a bonding
region disposed between the silicon material and the optical
detector, wherein the bonding region comprises: a metal-assisted
bond at a first portion of the bonding region, wherein the
metal-assisted bond includes an interface layer positioned between
the silicon material and the optical detector; and a direct
semiconductor-semiconductor bond at a second portion of the bonding
region; and control electronics disposed in the substrate and
electrically coupled to the set of optical transmitters and the set
of optical receivers.
2. The opto-electronic integrated circuit of claim 1 further
comprising a phase adjustment section optically coupled between the
waveguide and the optical coupler.
3. The opto-electronic integrated circuit of claim 1 wherein the
optical modulator comprises a Mach Zehnder modulator.
4. The opto-electronic integrated circuit of claim 1 wherein the
silicon material comprises a silicon on insulator wafer.
5. The opto-electronic integrated circuit of claim 4 wherein the
silicon on insulator wafer comprises a silicon substrate, an oxide
layer disposed on the silicon substrate, and a silicon layer
disposed on the oxide layer, wherein the first wavelength selective
element, the second wavelength selective element, and the waveguide
are disposed in the silicon layer.
6. The opto-electronic integrated circuit of claim 1 wherein: the
first wavelength selective element comprises a first index of
refraction adjustment device; and the second wavelength selective
element comprises a second index of refraction adjustment
device.
7. The opto-electronic integrated circuit of claim 6 wherein: the
first index of refraction adjustment device comprises a thermal
device; and the second index of refraction adjustment device
comprises a thermal device.
8. The opto-electronic integrated circuit of claim 1 wherein: the
first wavelength selective element comprises a first modulated
grating reflector; and the second wavelength selective element
comprises a second modulated grating reflector.
9. The opto-electronic integrated circuit of claim 8 wherein the
first modulated grating reflector comprises a superstructure
grating characterized by a first wavelength spacing between
modes.
10. The opto-electronic integrated circuit of claim 9 wherein the
second modulated grating reflector comprises a superstructure
grating characterized by a second wavelength spacing between modes
different than the first wavelength spacing between modes.
11. An opto-electronic integrated circuit comprising: an SOI
substrate including a silicon substrate, an oxide layer disposed on
the silicon substrate, and a silicon layer disposed on the oxide
layer; a set of composite optical transmitters, each of the
composite optical transmitters including: a waveguide disposed in
the silicon layer; a gain medium optically coupled to the silicon
layer, wherein the gain medium includes a compound semiconductor
material; an output mirror; and an optical modulator optically
coupled to the output mirror; and a set of composite optical
receivers, each of the composite optical receivers including: a
waveguide disposed in the silicon layer; an optical detector bonded
to the silicon layer, wherein the optical detector includes a
compound semiconductor material; and a bonding region disposed
between the silicon layer and the optical detector, wherein the
bonding region comprises: a metal-assisted bond at a first portion
of the bonding region, wherein the metal-assisted bond includes an
interface layer positioned between the silicon layer and the
optical detector; and a direct semiconductor-semiconductor bond at
a second portion of the bonding region; and control electronics
disposed in the substrate and electrically coupled to the set of
composite optical transmitters and the set of composite optical
receivers.
12. The opto-electronic integrated circuit of claim 11 wherein the
optical detector comprises at least one of an APD detector or a PIN
detector.
13. The opto-electronic integrated circuit of claim 11 wherein a
thickness of the interface layer is less than 100 .ANG..
14. The opto-electronic integrated circuit of claim 11 further
comprising a phase adjustment section optically coupled between the
waveguide and the optical coupler.
15. The opto-electronic integrated circuit of claim 11 wherein the
optical modulator comprises a Mach Zehnder modulator.
16. The opto-electronic integrated circuit of claim 11 wherein the
interface layer comprises In.sub.xPd.sub.y.
17. The opto-electronic integrated circuit of claim 16 wherein
x=0.7 and y=0.3.
18. The opto-electronic integrated circuit of claim 11 wherein each
of the composite optical transmitters include: a first wavelength
selective element optically coupled to the gain medium and
including a first modulated grating reflector; and a second
wavelength selective element optically coupled to the gain medium
and including a second modulated grating reflector.
19. The opto-electronic integrated circuit of claim 18 wherein the
first modulated grating reflector comprises a superstructure
grating characterized by a first wavelength spacing between
modes.
20. The opto-electronic integrated circuit of claim 19 wherein the
second modulated grating reflector comprises a superstructure
grating characterized by a second wavelength spacing between modes
different than the first wavelength spacing between modes.
Description
BACKGROUND OF THE INVENTION
Advanced electronic functions such as photonic device bias control,
modulation, amplification, data serialization and de-serialization,
framing, routing, and other functions are typically deployed on
silicon integrated circuits. A key reason for this is the presence
of a global infrastructure for the design and fabrication of
silicon integrate circuits that enables the production of devices
having very advanced functions and performance at market-enabling
costs. Silicon has not been useful for light emission or optical
amplification due to its indirect energy bandgap. This deficiency
has prevented the fabrication of monolithically integrated
opto-electronic integrated circuits on silicon.
Compound semiconductors such as indium phosphide, gallium arsenide,
and related ternary and quaternary materials have been extremely
important for optical communications, and in particular light
emitting devices and photodiodes, because of their direct energy
bandgap. At the same time, integration of advanced electrical
functions on these materials has been limited to niche,
high-performance applications due to the much higher cost of
fabricating devices and circuits in these materials.
Thus, there is a need in the art for improved methods and systems
related to hybrid integration of silicon and compound semiconductor
devices.
SUMMARY OF THE INVENTION
Embodiments of the present invention relate to hybrid-integrated
silicon photonics. More particularly, embodiments of the present
invention relate to an apparatus and method of hybrid integration
of compound semiconductor chips with tuning elements monolithically
integrated onto a silicon base and the like.
According to an embodiment of the present invention, techniques
related to photonic integration are provided. Merely by way of
example, embodiments of the present invention have been applied to
methods and systems for fabricating and operating a tunable laser
utilizing a hybrid design. More particularly, an embodiment of the
present invention includes a hybrid system including a
semiconductor laser device fabricated in a first material system
and a wavelength tuning device fabricated in a second material
system. In some embodiments, the tunable laser is fabricated using
bonding methodology described in related U.S. patent application
Ser. No. 12/902,621. However, the scope of the present invention is
broader than this application and includes other photonic
systems.
According to an embodiment of the present invention, an
opto-electronic integrated circuit is provided. The opto-electronic
integrated circuit includes a substrate comprising a silicon
material and a set of optical transmitters. Each of the set of
optical transmitters includes a gain medium coupled to the
substrate. The gain medium includes a compound semiconductor
material. Each of the set of optical transmitters also includes an
optical modulator optically coupled to the gain medium, a waveguide
disposed in the substrate and optically coupled to the gain medium,
a first wavelength selective element characterized by a first
reflectance spectrum and disposed in the substrate, a second
wavelength selective element characterized by a second reflectance
spectrum and disposed in the substrate, an optical coupler disposed
in the substrate and joining the first wavelength selective
element, the second wavelength selective element, and the
waveguide, and an output mirror.
The opto-electronic integrated circuit also includes a set of
optical receivers. Each of the set of optical receivers includes a
waveguide disposed in the silicon material, an optical detector
bonded to the silicon material, and a bonding region disposed
between the silicon material and the optical detector. The bonding
region includes a metal-assisted bond at a first portion of the
bonding region. The metal-assisted bond includes an interface layer
positioned between the silicon material and the optical detector.
The bonding region also includes a direct
semiconductor-semiconductor bond at a second portion of the bonding
region. The opto-electronic integrated circuit further includes
control electronics disposed in the substrate and electrically
coupled to the set of optical transmitters and the set of optical
receivers.
According to another embodiment of the present invention, an
opto-electronic integrated circuit is provided. The opto-electronic
integrated circuit includes an SOI substrate including a silicon
substrate, an oxide layer disposed on the silicon substrate, and a
silicon layer disposed on the oxide layer. The opto-electronic
integrated circuit also includes a set of composite optical
transmitters. Each of the composite optical transmitters includes a
waveguide disposed in the silicon layer and a gain medium optically
coupled to the silicon layer. The gain medium includes a compound
semiconductor material. Each of the optical transmitters also
includes an output mirror and an optical modulator optically
coupled to the output mirror.
The opto-electronic integrated circuit further includes a set of
composite optical receivers. Each of the composite optical
receivers includes a waveguide disposed in the silicon layer and an
optical detector bonded to the silicon layer. The optical detector
includes a compound semiconductor material. Each of the composite
optical receivers also includes a bonding region disposed between
the silicon layer and the optical detector. The bonding region
includes a metal-assisted bond at a first portion of the bonding
region. The metal-assisted bond includes an interface layer
positioned between the silicon layer and the optical detector. The
bonding region also includes a direct semiconductor-semiconductor
bond at a second portion of the bonding region. The opto-electronic
integrated circuit additionally includes control electronics
disposed in the substrate and electrically coupled to the set of
composite optical transmitters and the set of composite optical
receivers.
Numerous benefits are achieved by way of the present invention over
conventional techniques. For example, embodiments of the present
invention provide methods and systems suitable for reducing the
size and power consumption of optical communications systems,
relaxing the requirements for stringent temperature control of the
devices, and improving the laser linewidth through minimizing
refractive index fluctuations in the device. These and other
embodiments of the invention along with many of its advantages and
features are described in more detail in conjunction with the text
below and attached figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a simplified plan view illustrating a hybrid integrated
tunable laser according to an embodiment of the present
invention;
FIG. 1B is a simplified cross-sectional view illustrating a hybrid
integrated tunable laser according to a particular embodiment of
the present invention;
FIG. 1C is a simplified cross-sectional view illustrating a hybrid
integrated tunable laser according to a specific embodiment of the
present invention;
FIG. 2A is a cross-sectional view at cross section A-A' as
illustrated in FIG. 1A;
FIG. 2B is a cross-sectional view at cross section B-B' as
illustrated in FIG. 1A;
FIG. 3A is a simplified perspective view of a waveguide including
grating elements according to an embodiment of the present
invention;
FIG. 3B is a simplified cross-sectional view at a high index
portion of the waveguide illustrated in FIG. 3A according to an
embodiment of the present invention;
FIG. 3C is a simplified cross-sectional view at a low index portion
of the waveguide illustrated in FIG. 3A according to an embodiment
of the present invention;
FIG. 3D is a contour plot illustrating a TE mode for the high index
portion of the waveguide illustrated in FIG. 3B;
FIG. 3E is a contour plot illustrating a TM mode for the high index
portion of the waveguide illustrated in FIG. 3B;
FIG. 3F is a contour plot illustrating a TE mode for the low index
portion of the waveguide illustrated in FIG. 3C;
FIG. 3G is a contour plot illustrating a TM mode for the low index
portion of the waveguide illustrated in FIG. 3C;
FIG. 4A illustrates a reflectance spectrum for a first modulated
grating reflector according to an embodiment of the present
invention;
FIG. 4B illustrates a reflectance spectrum for a second modulated
grating reflector according to an embodiment of the present
invention;
FIG. 4C illustrates an overlay of the reflectance spectra shown in
FIG. 4A and FIG. 4B;
FIG. 4D illustrates constructive interference between the
reflectance spectra shown in FIG. 4A and FIG. 4B;
FIG. 5A is a plot illustrating operating wavelength as a function
of temperature change according to an embodiment of the present
invention;
FIG. 5B illustrates wavelength shifting of a reflectance spectrum
as a function of index of refraction according to an embodiment of
the present invention;
FIG. 6 is a simplified flowchart illustrating a method of operating
a hybrid integrated laser according to an embodiment of the present
invention;
FIG. 7 is a simplified plan view illustrating a hybrid integrated
tunable laser and an optical modulator according to an embodiment
of the present invention;
FIG. 8A is a simplified plan view illustrating a composite-bonded
die joined to a silicon photonic substrate according to an
embodiment of the present invention;
FIG. 8B is a simplified side view of the composite-bonded die and
silicon photonic substrate illustrated in FIG. 8A;
FIG. 8C is a magnified view of the interface between the
composite-bonded die and silicon photonic substrate illustrated in
FIG. 8B; and
FIG. 9 illustrates an opto-electronic integrated circuit (OIEC)
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Hybrid integration on silicon is preferable for the commercial
deployment of optoelectronic integrated circuits. Silicon is a
preferable material for electronic integration. Silicon technology
has advanced such that extremely complex electronic functions can
be realized very inexpensively. Silicon is also a good material for
constructing low loss optical waveguides. However, monolithic
integration of light generating or detecting functions has been
prevented in silicon because it is an indirect bandgap material.
Conversely, compound semiconductor materials, including III-V
materials such as indium phosphide are well suited for light
generation and detection because of their physical properties such
as being direct bandgap materials. These materials are complex
material systems with small substrates and relatively (compared to
silicon) low yields. As such, constructing devices with a high
level of functionality is currently cost prohibitive.
Embodiments of the present invention relate to an apparatus and
method for hybrid integration of compound semiconductor devices
with tuning elements monolithically integrated onto a silicon base
or similar material. Throughout this specification, the term
"composite integration" can be used interchangeably with the term
"hybrid integration." Preferably, hybrid or composite integration
is the method to overcome the specific deficiencies of silicon and
compound semiconductors while capitalizing on their respective
strengths. Embodiments of the present invention preferably utilize
the complex electronic functionality available using silicon
devices to minimize cost, and the optical functions (e.g., light
generation and detection) available using III-V materials to form
hybrid integrated systems. Some embodiments of the present
invention remove functionality from the III-V material system and
transfer such functionality to the silicon system to improve system
performance.
Embodiments of the present invention utilize photonic apparatus
fabricated using compound semiconductor material systems that are
mounted onto silicon integrated circuit platforms and the like.
Embodiments of the present invention achieve photonic integration
by utilizing a plurality of techniques and apparatus that do not
historically rely on a direct energy bandgap, including, but not
limited to, waveguides, optical multiplexers, optical
demultiplexers, optical modulators, and the like, that can be
fabricated using silicon and similar materials. Embodiments of the
present invention optionally include, but are not limited to,
methods of modifying the refractive index of silicon via current
injection or local heating.
Embodiments of the present invention include, but are not limited
to, optionally utilizing the laser devices that serve as the
initial source of optical energy. In today's dense wavelength
division multiplexing ("DWDM") systems, the laser sources are
typically fixed-wavelength distributed feedback lasers or tunable
lasers. Tunable lasers preferably provide additional flexibility to
the optical communications network operators. Some DWDM systems can
use lasers with up to 80 different wavelengths. A single tunable
laser is capable of tuning to any of those wavelengths. One tunable
laser can be inventoried and used to replace any of 80 fixed
wavelength lasers, thereby reducing the required inventory levels
and the associated costs.
The term "silicon" as used throughout this application includes but
is not limited to tetravalent nonmetallic elements and the like.
The term "laser" as used throughout the specification includes but
is not limited to an acronym for light amplification by stimulated
emission of radiation; and/or an optical device that produces an
intense monochromatic beam of coherent light. The term "SOI" and/or
"Silicon on Insulator" stands for, a type of substrate material as
used throughout this specification includes but is not limited to
grating and tuning testing. The term "DWDM" and/or "Dense
Wavelength Division Multiplexing" as used throughout this
application includes but is not limited to a technique utilized by
the optical communications industry to maximize system bandwidth
while minimizing capital expenditures and operational expenditures.
These costs are minimized through the use of DWDM techniques
because the system operators can increase their system bandwidth
simply by adding another optical wavelength as opposed to needing
to deploy additional optical fibers which usually requires
significant expense. The term "bandgap" as used throughout this
application includes but is not limited to an energy range in a
solid where no electron states exist; and/or the energy difference
between the top of the valence band and the bottom of the
conduction band; and/or the amount of energy required to free an
outer shell electron from its orbit about the nucleus to a free
state; and/or any combination thereof. The term "photonic
integration" as used throughout this application includes but is
not limited to the meaning to make into a whole or make part of a
whole multiple functions and reduce packaging size by an order of
magnitude, for example, while matching the performance of a
subsystem built with discrete components. The term "gain media" and
interchangeably "gain chip" as used throughout this application
includes but is not limited to the source of optical gain within a
laser. The gain generally results from the stimulated emission of
electronic or molecular transitions to a lower energy state from a
higher energy state. The term "InP" or "Indium Phosphide", as used
throughout this application is used interchangeably with the phrase
"III-V compound semiconductor".
FIG. 1A is a simplified plan view illustrating a hybrid integrated
tunable laser according to an embodiment of the present invention.
As illustrated in FIG. 1A, laser 10 is a hybrid integrated
structure including both active and passive elements disposed on or
fabricated in a silicon substrate 22. Although a silicon substrate
22 is illustrated, this is intended to include a variety of
semiconductor devices fabricated using the silicon material system.
Such devices include CMOS circuitry, current sources, laser
drivers, thermal system controllers, passive optical elements,
active optical elements, and the like.
Referring to FIG. 1A, a first modulated grating reflector 12 and a
second modulated grating reflector 14 are fabricated on the silicon
substrate 22. Modulated grating reflectors 12 and 14 are preferably
modifiable to adjust the refractive index. The first modulated
grating reflector 12 and the second modulated grating reflector 14
are examples of wavelength selective elements that are utilized
according to embodiments of the present invention. The illustration
of the use of modulated grating reflectors in FIG. 1A is not
intended to limit the scope of the present invention but merely to
provide examples of wavelength selective elements. Other wavelength
selective elements can be utilized in embodiments of the present
invention. As described more fully below, the wavelength selective
elements can be sampled Bragg gratings or sampled distributed
feedback reflectors that provide a comb of reflectance peaks having
a variable comb spacing over a tunable wavelength range.
Embodiments of the present invention are not limited to these
implementations and photonic crystals, etalon structures, MEMS
devices, ring resonators, arrayed-waveguide grating devices,
Mach-Zehnder lattice filters, and the like can be employed as
wavelength selective elements. A benefit provided by the wavelength
selective elements discussed herein is a reflection spectra
including a single or multiple peaks that can be shifted through
the use of a controllable parameter such as current, voltage,
temperature, mechanical force, or the like.
As an example, heaters integrated into the silicon substrate can be
utilized to locally change the temperature of the region
surrounding the modulated grating reflectors and thereby, the index
of refraction. As described more fully below, the ability to
control the local index of refraction provides the functionality of
varying the reflectivity of the modulated grating reflectors and
the output wavelength of the hybrid integrated device.
Laser 10 further includes, but is not limited to, multimode
interference coupler 16 and one or multiple phase adjustment
sections 18. The phase adjustment section 18 can also be referred
to as a phase control region that provides for correction of phase
misalignment between the output of the coupler section, which may
be implemented through wavelength selective devices (e.g., the
grating sections) and the gain media 20. In the illustrated
embodiment, the phase adjustment section 18 is positioned between
the multimode interference coupler 16 and the gain media 20,
however, other embodiments locate this element in different
locations providing the same or similar performance
characteristics.
The coupler section, which may be implemented through the use of a
multimode interference coupler, y-branch, or other method, splits
and recombines light from two or more tuning sections. The
multimode interference coupler, which is based upon the principle
that coherent light launched from a waveguide (input waveguide)
into a propagation section will self image at periodic intervals,
can be used to efficiently achieve n.times.m splitting ratios. In
this instance, the design is optimized for a 1.times.2 split but
other splitting ratios may be employed in the case where there are
either multiple gain chips or more than 2 tuning arms. An advantage
provided by the illustrated device is that coherent light returning
from the tuning arms, where the phase relationship of the light is
fixed, can be coupled back into the launch waveguide with minimal
excess loss. In order to ensure that the interference pattern of
the returning light has maximum overlap with the input waveguide, a
phase adjustment section may be implemented in one or more of the
branch arms. In addition to phase adjustment in the branch arms, a
phase adjustment section 18 is utilized in the waveguide section
leading from the coupler 16 to the gain chip 20. This phase
adjustment section, which can be implemented though a device such
as a heater or current injection electrode, which changes the
refractive index in the waveguide layer under the device, serves to
provide an overlap between the cavity modes of the device and the
grating mode selected by tuning section.
As illustrated in FIG. 1A, gain media 20 (also referred to as a
gain chip) fabricated using a compound semiconductor material
system is integrated with the silicon substrate 22 in a hybrid
configuration. The compound semiconductor material, which is direct
bandgap, provides optical gain for the laser device. The hybrid
integration or attachment of the gain media (and/or other compound
semiconductor elements) to the silicon substrate can be provided in
one or several manners. In a particular embodiment, the hybrid
integration is performed using the methods and systems described in
the related application reference in paragraph [0002]. In addition
to gain media, absorptive media fabricated using compound
semiconductor materials can be integrated with the silicon
substrate. Embodiments of the present invention integrate III-V
devices and structures acting as gain and/or absorption regions
with silicon photonics elements in which optical and/or electrical
functionality is provided. The silicon photonic elements may
include CMOS circuitry and the like. One of ordinary skill in the
art would recognize many variations, modifications, and
alternatives.
As discussed in more detail in relation to FIGS. 4-4D, modulated
grating reflectors 12 and 14 provide feedback at one end of the
laser 10. Feedback in the form of a front facet reflector is
provided by a low reflectance coating (e.g., a dielectric coating
with a reflectance of a few percent, for example, .about.1-10%)
applied to the gain media on surface 21. Alternatively, a
distributed feedback (e.g., a grating) structure could be
integrated into the silicon substrate to provide feedback for the
laser cavity. In another embodiment, a low reflectance coating is
applied to a surface of the silicon substrate. One of ordinary
skill in the art would recognize many variations, modifications,
and alternatives. As illustrated in FIG. 1A, optical functionality
other than optical gain has been transferred from the III-V
materials in which it is typically located and integrated into the
silicon materials, thereby increasing device yield in comparison
with designs that are fully integrated in III-V materials. In the
illustrated embodiment, the tunable reflective sections (also
referred to as wavelength selective devices) and other optical
functions are performed in the silicon material.
FIG. 1A also illustrates heater element 26 and temperature sensor
28 associated with first modulated grating reflector 12 and heater
element 27 and temperature sensor 29 associated with the second
modulated grating reflector. In an embodiment, the heater element
can be a thin film resistor formed through the vacuum deposition of
a material such as W, NiCr, TaN, WSi, RuO.sub.2, PbO,
Bi.sub.2Ru.sub.2O.sub.7, Bi.sub.2Ir.sub.2O.sub.7, cobalt salicide,
or the like.
In an embodiment, the temperature sensor can be a resistive thermal
device (RTD), a thermocouple, a p-n junction, or the like. By
flowing a current through the heaters, the temperature of the
region surrounding the modulated grating reflectors can be modified
in order to modify the index of refraction and the reflectance
profile as a result. Phase adjustment section 18, which also may
use the temperature dependence of the refractive index to control
the effective optical length and thereby the phase of light, is
also provided with a heater and a temperature sensor to provide
similar functionality and wavelength tunability.
Some embodiments of the present invention utilize thermal tuning to
achieve index of refraction changes in the silicon-based modulated
grating reflectors. One of the benefits available using thermal
tuning is a significant reduction in the short time scale
variations in index of refraction that are produced using thermal
tuning in comparison to these variations achieved using current
tuning in the InP or GaAs material system. Such improvement in
refractive index stability will result in a laser linewidth
significantly narrower than can be achieved using other approaches.
As will be evident to one of skill in the art, the stable tuning
provided by embodiments of the present invention enables use of the
lasers described herein in DWDM applications and other applications
utilizing precisely tuned lasers. As an example, advanced
modulation techniques such as DQPSK can benefit from use of the
lasers described herein.
The phase adjustment section operates through the modification of
the refractive index of the waveguide section contained therein.
Through modification of the refractive index, the phase angle of
the light exiting the phase adjustment device relative the input
phase angle can be precisely controlled. This allows the alignment
of laser cavity modes with grating modes. In the illustrated
embodiment, the phase adjustment device 18 includes a heater 19 and
a temperature sensor (e.g., an RTD) 17.
FIG. 1B is a simplified cross-sectional view illustrating a hybrid
integrated tunable laser according to a particular embodiment of
the present invention. As illustrated in FIG. 1B, direct coupling
between the waveguide in the gain media and the waveguide in the
silicon layer is utilized. The heater element and the temperature
sensor (e.g., an RTD) are illustrated for the phase adjustment
section as well as the modulated grating reflector sections. An
encapsulant is illustrated over the modulated grating reflector
sections. The encapsulant provides for electrical isolation among
other features.
FIG. 1C is a simplified cross-sectional view illustrating a hybrid
integrated tunable laser according to a specific embodiment of the
present invention. The structure illustrated in FIG. 1C is similar
to that illustrated in FIG. 1B except that evanescent coupling
between the waveguide in the gain media and the waveguide in the
silicon layer is utilized.
Referring to FIG. 1B, a Controlled Index Layer is illustrated that
is not necessarily the same as the index matching layer illustrated
in FIG. 2B. The controlled index layer can be used for mode shaping
in the silicon waveguide, for example, by using air, SiO.sub.2 or
the like. According to some embodiments of the present invention, a
higher index material is utilized to broaden the mode in the
silicon waveguide such that optical coupling to the gain media is
improved. If the controlled index layer is not an insulator, an
encapsulant layer may also be used between the heater metal and
controlled index layer. As illustrated in FIGS. 1B and 1C, either
direct coupling (also known as butt coupling) or evanescent
coupling of the gain media to the silicon waveguide may be
used.
Referring to FIG. 1C, the optical coupler, which may be a device
such as a MMI (multimode interference coupler) is illustrated. In
some embodiments, an MMI can be formed using an unguided
propagation region. Additionally, although not illustrated in FIGS.
1A-1C, a second phase adjust region may be provided in one of the
legs of the Y-branched structure in addition to the phase
adjustment section illustrated at the output of the tuning
section.
FIG. 2A is a cross-sectional view at cross section A-A' as
illustrated in FIG. 1A. The silicon substrate 22 is illustrated as
well as a silicon-on-insulator (SOI) oxide layer 23 and an SOI
silicon layer 24. In the embodiment shown, a portion of the SOI
silicon layer has been removed using an etching or other process to
provide a recessed region into which the gain chip has been
inserted. Such etching may not be performed in the case where
evanescent coupling of the light from the gain chip into the
silicon waveguide is used. The gain chip is bonded to the silicon
substrate in the embodiment illustrated in FIG. 2A using a
metal/metal structural bond at locations 25 that provide an
electrical bond between the hybrid elements. Additionally, a
metal/semiconductor or a semiconductor/semiconductor bond is
illustrated. Combinations of these bonding techniques can be
implemented as well. One of ordinary skill in the art would
recognize many variations, modifications, and alternatives.
FIG. 2B is a cross-sectional view at cross section B-B' as
illustrated in FIG. 1A. As will be evident to one of skill in the
art, the optical waveguide in the gain chip will be coupled to an
optical waveguide in the SOI silicon layer. An index matching
region is provided at the interface between the gain chip and the
SOI silicon layer to facilitate a high degree of optical coupling
between the hybrid devices and to reduce or minimize parasitic
reflections. The index matching region can be filled with an
appropriate index matching material, remain empty, have optical
coatings applied to the surfaces of the hybrid devices as
illustrated at facets 26 and/or 27, combinations thereof, or the
like.
Referring once again to FIG. 1A first modulated grating reflector
12 provides optical feedback creating a comb of reflected optical
wavelengths. Second modulated grating reflector 14 provides optical
feedback characterized by a different optical period, thereby
resulting in a variable set of reflected wavelengths. The two combs
of wavelengths are combined in optical coupler 16. The combs
overlap and lasing preferably occurs due to constructive
interference. Optionally, where the combs do not overlap, lasing is
preferably prevented due to destructive interference. Specific
optical spectra of first modulated grating reflector 12 and/or
second modulated grating reflector 14 can be modified by varying
the refractive index. The refractive index is preferably modified
by varying the temperature of the modulated grating reflectors 12,
14 using a heating element. The amount of heating is optionally
monitored through use of an RTD element.
Phase adjustment is provided using phase adjustment region 18 to
compensate for small phase offsets between the reflection spectra
from first modulated grating reflector 12 and the second modulated
grating reflector 14. Embodiments of the present invention comprise
functional blocks that can be realized in a compound semiconductor
such as indium phosphide, and/or silicon and/or similar material.
Embodiments of the present invention comprise tuning by modifying
the refractive index of the silicon and the like, preferably using
a thermal technique.
In embodiments of the present invention, the gain media, which
preferably uses a direct-bandgap material, can be realized in a
compound semiconductor material. Other embodiments of the present
invention include functional blocks that can be realized in silicon
material systems. Embodiments of the present invention utilize a
hybrid-approach that is preferable for a variety of reasons that
include, but are not limited to: manufacturing components using
methods that can result in high-yields at low cost; virtually
unlimited levels of additional integration can be achieved because
of the complexity of the III-V material system as compared to the
Si material system, and the like. Therefore, embodiments of the
present invention encompass substantially all necessary circuits to
control the operation of the tunable laser and can also be
monolithically integrated with silicon-based devices.
It should be noted that while embodiments of the present invention
have been implemented in relation to products produced by the
semiconductor industry, embodiments of the present invention are
also useful in optical communications networks for the
telecommunications industry, the enterprise communications
industry, high-performance computing interconnects, back-plane
optical interconnects, chip-to-chip optical interconnects,
intra-chip optical interconnects, and the like. In addition to
these communication applications, embodiments of the present
invention also have applications in the medical device
industry.
The following figures illustrate an analysis and applications of
waveguides created in silicon using an SOI substrate with a silicon
dioxide cap layer. This material system is merely described by way
of example and embodiments of the present invention can be
implemented in other material systems.
FIG. 3A is a simplified perspective view of a waveguide according
to an embodiment of the present invention. As illustrated in FIG.
3A, a waveguide structure is formed with a periodic variation in
thickness of one or more layers making up the waveguide. In the
illustrated embodiment, the SOI silicon layer varies in thickness
with a high portion having thickness H and a low portion having
thickness H-h. The width of the waveguide is W. For purposes of
clarity, only the top two SOI layers (i.e., the SOI oxide layer and
the SOI silicon layer) are illustrated in FIGS. 3A-3C. FIG. 3B is a
simplified cross-sectional view at a high index portion of the
waveguide illustrated in FIG. 3A according to an embodiment of the
present invention. FIG. 3C is a simplified cross-sectional view at
a low index portion of the waveguide illustrated in FIG. 3A
according to an embodiment of the present invention. It should be
noted that the top SiO.sub.2 layer shown in these figures may be
replaced by another index-controlled layer such as air, TiO.sub.2,
SiC, ZnS, Nb.sub.2O.sub.5, HfO.sub.2, ZrO.sub.2. As will be evident
to one of skill in the art, the indexes of the various materials
will impact the shape of the optical modes.
The waveguide structure was analyzed to determine an effective
index for the various sections of the waveguide. A vector EM mode
solver was used and applied to two different single mode ridge
waveguides with two different ridge heights. The effective indices
n.sub.H and n.sub.L and mode profiles could be extracted, then the
full three-dimensional problem was a one-dimensional problem, with
the one-dimensional transfer matrix method efficiently simulating
the multi-layer structures. The index difference created
reflections that accumulated coherently over the length result in
differing reflectances versus wavelength.
FIG. 3D is a contour plot illustrating a TE mode for the high index
portion of the waveguide illustrated in FIG. 3B. FIG. 3E is a
contour plot illustrating a TM mode for the high index portion of
the waveguide illustrated in FIG. 3B. FIG. 3F is a contour plot
illustrating a TE mode for the low index portion of the waveguide
illustrated in FIG. 3C. FIG. 3G is a contour plot illustrating a TM
mode for the low index portion of the waveguide illustrated in FIG.
3C.
FIG. 4A illustrates a reflectance spectrum for a first modulated
grating reflector according to an embodiment of the present
invention and FIG. 4B illustrates a reflectance spectrum for a
second modulated grating reflector according to an embodiment of
the present invention. As illustrated in FIG. 4A, the grating
structure includes a superstructure grating (SSG) in which
periodically modulated gratings provide a comb-like reflection
spectrum. In these gratings, multiple elements of periodicity are
provided such that the mode spacing associated with the grating is
overlaid with an envelope. The spacing between the modes of the
comb will be a function of the height and other features of the
grating features formed in the waveguide.
As an example of an SSG, the reflectance spectrum illustrated in
FIG. 4A was obtained using the following 3-step modulated
superstructure grating parameters:
Duty cycles=[0.5 0.5 0.5]
Periods=[227.7 230 232.3] nm
N.sub.sub=[110 109 108]
.LAMBDA..sub.s=(25.047+25.07+25.088)=75.205.mu.,
n.sub.H=3.3757; n.sub.L=3.3709;
.DELTA.n=n.sub.H-n.sub.L=0.0048
N.sub.p=11
Total number of periods=3597 mixed periods
For these grating parameters, a mode spacing of
.DELTA..lamda..sub.1=4.7 nm was achieved.
As another example of a SSG, the reflectance spectrum illustrated
in FIG. 4B was obtained using the following 3-step modulated
superstructure grating parameters:
Duty cycles=[0.5 0.5 0.5]
Periods=[228.2 230 231.8] nm
N.sub.sub=[131 130 129]
.LAMBDA.s=(29.894+29.9+29.902)=89.696 .mu.m
n.sub.H=3.3757; n.sub.L=3.3709;
.DELTA.n=n.sub.H-n.sub.L=0.0048
N.sub.p=11
Total number of periods=4290 mixed periods
For these grating parameters, a mode spacing of
.DELTA..lamda..sub.2=4.0 nm was achieved.
FIG. 4C illustrates an overlay of the reflectance spectra shown in
FIG. 4A and FIG. 4B. FIG. 4D illustrates constructive interference
between the reflectance spectra shown in FIG. 4A and FIG. 4B. The
first and second modulated grating reflectors are designed to
provide different peak spacings such that only a single peak is
aligned. Thus, only one cavity mode is selected for lasing. As
described below, the single peak can be widely tuned over
wavelength space based on thermal effect, free carrier injection,
or the like. Although embodiments of the present invention are
illustrated in relation to operation and tunability around 1550 nm,
other wavelengths are available using appropriate semiconductor
laser materials.
Thus, implementations of the silicon hybrid tunable laser of the
present invention was capable of tuning over the substantially
entire wavelength range of interest. Tuning can be achieved, as
described more fully below using several techniques including
thermal tuning Referring once again to FIGS. 4A and 4B, the
illustrated embodiment is operable over a range of temperatures
including 40.degree. C. Tuning of the laser wavelength can be
considered as follows: the comb of wavelengths illustrated in FIG.
4A is created by the first modulated grating reflector 12
illustrated in FIG. 1A. The comb of wavelengths illustrated in FIG.
4B is created by the second modulated grating reflector 14
illustrated in FIG. 4B. The overlay of the first comb and the
second comb is illustrated in FIG. 4C and demonstrates the
combination of the wavelengths obtained from the first modulated
grating reflector 12 and the second modulated grating reflector 14.
The constructive interference between the two wavelength combs is
illustrated in FIG. 4D, with substantially a single peak in the
reflectance profile. The one strong reflection peak thus produces
the single laser mode, which is the only mode supported by the
combined reflectances. In an embodiment, the spectrum illustrated
in FIG. 4D will be present as the output of the optical coupler 16
provided to the phase adjustment section 18.
FIG. 5A is a plot illustrating operating wavelength as a function
of temperature change according to an embodiment of the present
invention. As illustrated in FIG. 5A, the operating wavelength
shifts as a function of temperature in a substantially linear
manner. As will be evident to one of skill in the art, the shift in
wavelength of the reflection peak as a function of temperature (and
index of refraction) results in the shift in operating
wavelength.
FIG. 5B illustrates wavelength shifting of a reflectance spectrum
as a function of index of refraction according to an embodiment of
the present invention. For a nominal index (.DELTA.n=0), the peaks
of the comb are located at a first set of wavelengths. As the index
of refraction is shifted, for example, by thermal tuning, the comb
shifts to a new set of wavelengths as illustrated by the combs
associated with .DELTA.n=0.003 and .DELTA.n=0.006. Thus,
embodiments of the present invention provide for tunability of
silicon photonics in which tuning is accomplished using the thermo
optic (TO) effect of silicon. The TO coefficient of silicon is
approximately C.sub.TO=2.4.times.10.sup.4K.sup.-1 over the
temperature range up to 650.degree. C. In the embodiments described
herein, a conventional silicon ridge waveguide was used for
waveguiding so that the TO is considered to be in the same range as
the value given above. The index of refraction due to the TO effect
can be expressed as: .DELTA.n=C.sub.TO.DELTA.T.
Thus, for a temperate change of about 40.degree. C., a change in
the index of refraction of about 0.0096 can be provided for silicon
material. As illustrated in FIG. 5B, this translates to a change of
about 4 nm in laser wavelength change. It should be noted that the
dynamic tuning range for each mode can be adjusted by increasing
the number of super-periods (N.sub.p).
In addition to thermal tuning, embodiments of the present invention
can utilize current tuning based on the Kramer-Kronig relation.
FIG. 6 is a simplified flowchart illustrating a method of operating
a hybrid integrated laser according to an embodiment of the present
invention. The method 600, which may be utilized in operating a
tunable laser, includes tuning a first wavelength selective device
(e.g., a first modulated grating reflector disposed in a silicon
layer of an SOI wafer) (610) and tuning a second wavelength
selective device (e.g., a second modulated grating reflector
disposed in the silicon layer of the SOI wafer) (612). The first
wavelength selective device is characterized by a first reflectance
spectra including a first plurality of reflectance peaks. The
second wavelength selective device is characterized by a second
reflectance spectra including a second plurality of reflectance
peaks. In a particular embodiment, a first modulated grating
reflector includes a superstructure grating characterized by a
first wavelength spacing between modes and a second modulated
grating reflector includes a superstructure grating characterized
by a second wavelength spacing between modes that is different than
the first wavelength spacing between modes. The wavelength
selective devices can include index of refraction adjustment
devices such as thermal devices that enable the tuning
functionality that is provided. In applications with thermal
devices, temperature sensors such as RTDs can be used to monitor
and control thermal inputs. One of ordinary skill in the art would
recognize many variations, modifications, and alternatives.
The method also includes generating optical emission from a gain
medium comprising a compound semiconductor material (614) and
waveguiding the optical emission to pass through an optical coupler
(616). The optical emission may pass through a phase adjustment
region. The method further includes reflecting a portion of the
optical emission having a spectral bandwidth defined by an overlap
of one of the first plurality of reflectance peaks and one of the
second plurality of reflectance peaks (618), amplifying the portion
of the optical emission in the gain medium (620), and transmitting
a portion of the amplified optical emission through an output
mirror (622).
It should be appreciated that the specific steps illustrated in
FIG. 6 provide a particular method of operating a hybrid integrated
laser according to an embodiment of the present invention. Other
sequences of steps may also be performed according to alternative
embodiments. For example, alternative embodiments of the present
invention may perform the steps outlined above in a different
order. Moreover, the individual steps illustrated in FIG. 6 may
include multiple sub-steps that may be performed in various
sequences as appropriate to the individual step. Furthermore,
additional steps may be added or removed depending on the
particular applications. One of ordinary skill in the art would
recognize many variations, modifications, and alternatives.
FIG. 7 is a simplified plan view illustrating a hybrid integrated
tunable laser and an optical modulator according to an embodiment
of the present invention. The combination of a tunable laser and a
modulator provides a tunable pulsed laser system 700. It will be
appreciated that the laser itself is typically operated CW and
modulation of the CW laser light by the optical modulator results
in intensity variation in the optical output. Thus, although the
present specification discusses pulsed laser operation, the overall
device can be considered as a pulsed laser, although possibly not
in the same way that a laser engineer might think of a "classical"
pulsed laser in which lasing action throughout the entire cavity is
suppressed. As illustrated in FIG. 7, an optical modulator 710 is
integrated with the hybrid-integrated tunable laser described
herein. After light passes through the gain chip 20, it is split
into the two legs 712 and 714 of the illustrated modulator, where
the light in one leg can be phase shifted with respect to the light
in the other leg, enabling modulation of the light to be
implemented. Elements to apply the phase shift to the light in one
leg with respect to the other leg, such as electrodes, conductors,
and the like, are not illustrated for the purpose of clarity.
Although a single optical modulator 710 is illustrated in FIG. 7,
one or more optical modulators can be utilized according to
embodiments of the present invention. Thus, embodiments of the
present invention are suitable for use with advanced modulation
formats. It will be appreciated that more complex modulator
structures for advanced modulation formats can be utilized in which
data is encoded on both the optical amplitude and/or phase of the
optical signal.
The optical modulator 710 illustrated in FIG. 7 is a Mach Zehnder
modulator, but other optical modulators utilizing other modulation
methods, for example, amplitude modulation using absorption effects
(e.g., the quantum-confined Stark effect) may be utilized according
to alternative embodiments of the present invention. Therefore,
although a Mach Zehnder modulator is illustrated in FIG. 7,
embodiments of the present invention are not limited to this
particular implementation.
Utilizing the fabrication methods described herein, the modulator
may be directly integrated into the silicon. In other embodiments,
materials other than silicon are used in implementing the modulator
and can be fabricated using composite bonding methods. Examples of
other materials suitable for inclusion in the modulator include
ternary or quaternary materials lattice-matched to InP or GaAs,
non-linear optical materials such as lithium niobate, or the
like.
In the embodiment illustrated in FIG. 7, the optical modulator can
modulate the output produced from output mirror 21 formed on the
gain chip (i.e., an external modulator) or can be operated as an
intracavity modulator (i.e., the optical modulator is an
intracavity optical element), with an output mirror provided at
surface 720. Thus, both external modulation and internal modulation
techniques are included within the scope of the present invention.
The output produced by the tunable pulsed laser is characterized by
a tunable wavelength and pulse characteristics associated with the
optical modulator. Utilizing the embodiments described herein, data
can be encoded on either or both the amplitude and/or phase of the
optical signal.
In an embodiment of the present invention, electronics are provided
to drive and control all or a subset of the optical devices with
electrical input or output signals. In other embodiments, further
optical devices with or without their associated electronics, such
as monitor photodiodes for various sections of the optical path,
are included on the silicon photonic chip. It should be noted that
the optical modulator 710 illustrated in FIG. 7 may be replaced
with a "modulator section" including multiple modulator elements
suitable for systems using advanced modulation formats. Merely by
way of example, in a more complex modulation technique, one of the
split legs may be rotated in polarization and parallel modulators
would then be used to encode information on both polarization
states prior to recombining. One of ordinary skill in the art would
recognize many variations, modifications, and alternatives.
FIG. 8A is a simplified plan view illustrating a composite-bonded
die joined to a silicon photonic substrate according to an
embodiment of the present invention. As illustrated in FIG. 8A, a
die 820, which can include a III-V material, a magnetic material,
or the like, is bonded to a silicon photonic substrate 810,
illustrated as an SOI substrate. In a particular embodiment, the
bonded die includes an avalanche photodiode (APD) waveguide
receiver suitable for use in an optical communications system. A
silicon waveguide 815 is formed in the silicon photonic substrate
810 as discussed throughout the present specification. As an
example, the SOI substrate can include a silicon substrate, an
oxide layer, and a silicon layer, which can be a single crystal
silicon layer. In some embodiments, the silicon waveguide 815 is
formed in the silicon layer (e.g., a single crystal silicon layer)
of the silicon photonic substrate 810. As described more fully
below, light propagating in the silicon waveguide 815 is coupled
into the bonded die, where the light is absorbed as part of a
detection process in some embodiments. Thus, embodiments of the
present invention provide a waveguide photodiode integrated onto a
silicon photonic IC substrate.
FIG. 8B is a simplified side view of the composite-bonded die and
silicon photonic substrate illustrated in FIG. 8A. FIG. 8B is a
simplified side view of the composite-bonded die and silicon
photonic substrate illustrated in FIG. 8A. In the waveguide
receiver illustrated in FIG. 8B, evanescent coupling is used to
couple light into the absorption region of the bonded die.
Referring to FIG. 8B, a light propagation region 830 characterized
by a first index of refraction (also referred to as a waveguide) is
positioned between cladding regions 832 and 834 characterized by a
second index of refraction less than the first index of refraction.
The cladding regions can be formed by etching trenches and then
filling the trenches with a planarizing material. In some
embodiments, the cladding regions include air or other suitable
gases. In other embodiments, an oxide or other suitable passivating
material is deposited and planarized to form the cladding regions.
Thus, one or more waveguides can be formed in silicon layer 812.
Light produced or received at another region of the device can thus
propagate with acceptable loss to the bonded die.
FIG. 8C is a magnified view of the interface between the
composite-bonded die and silicon photonic substrate illustrated in
FIG. 8B. The expanded view of the interface between the bonded die
and the silicon shows a metal layer that has been patterned using a
predetermined pattern to allow for a combination of direct
semiconductor bonding and metal-assisted bonding as described in
related U.S. patent application Ser. No. 12/902,621, previously
incorporated by reference.
Bond 840 is a direct semiconductor/semiconductor bond between the
material of the die (e.g., a semiconductor material) and the
silicon material of silicon layer 812. The direct
semiconductor/semiconductor bond can be formed using techniques
including either chemical activation or plasma activation of the
surfaces and joining the materials together with pressure and low
temperature in order to bond the two surfaces together. Direct
semiconductor bonding is useful in devices employing evanescent
coupling in a waveguide structure as it will have lower optical
attenuation than metal-assisted semiconductor bonding. As
illustrated in FIG. 8C, the direct bond is formed at a location at
which the mode propagating in the waveguide is a maximum, thereby
providing a low loss waveguide suitable for a variety of
applications. Evanescent coupling enables the light propagating in
the waveguide 830 to couple through the direct bond region into the
bonded die, enabling, for example, detector applications.
Bond 842 is a metal-assisted semiconductor/semiconductor bond. For
the metal-assisted semiconductor/semiconductor bond, a thin metal
layer 844 (e.g., ranging from one to a few monolayers to a few tens
of monolayers) is deposited and patterned to improve the robustness
of the interface and to better accommodate the CTE differences
between silicon of silicon layer 812 and the compound semiconductor
material of the die. In an embodiment, the thin metal layer (e.g.,
an InPd-based metal layer) is less than 50 .ANG. in thickness. The
very thin interfacial metal will allow light to propagate through
the metal layer (i.e., in a vertical direction in FIG. 8C) with
limited attenuation. However, light propagating in a direction
parallel to the interface between the bonded die and the silicon
layer (i.e., into the plane of the image) will experience greater
propagation losses. As will be evident to one of skill in the art,
the waveguide 830 will support a fundamental mode with a
substantially Gaussian mode profile. Thus, embodiments of the
present invention utilize metal-assisted bonds outside the
waveguide and partial metal-assisted bonds in the waveguide region.
The partial metal-assisted bond includes a first portion with
little attenuation (and greater transparency) due to the direct
bond adjacent the center of the fundamental mode and a second
portion with acceptable attenuation in the wings of the mode
underlying the patterned metal layer 844. As discussed above, the
optical loss in the direct bond regions is less than in the
metal-assisted bonded regions. Thus, system designers can utilize
the patterning of the metal layer to provide sufficient bond
strength in the metal-assisted bonding areas and low optical loss
in the direct bond regions as appropriate to the particular
application.
In some embodiments, evanescent coupling can also be used with
higher order modes, including more than one maximum in the electric
field and intensity profile of the higher order mode. In these
embodiments, a node in the electric field of the higher order mode
would be aligned with the bond interface, which is characterized by
higher loss that other portions of the waveguide. Operation of the
higher order mode with such a field distribution would reduce
propagation losses because the field intensity would be lower in
the high-loss region. One of ordinary skill in the art would
recognize many variations, modifications, and alternatives.
Utilizing the combination of direct semiconductor/semiconductor
bonds and metal-assisted semiconductor/semiconductor bonds, a
hybrid bonding approach is provided that features the benefits of
both types of bonds, thereby reducing or overcoming the
disadvantages of low temperature semiconductor/semiconductor
bonding including the weak interface. Thus, embodiments of the
present invention provide for high strength bonds (metal-assisted
bond 842) while enabling low optical loss in regions of the
structure suitable for light propagation (direct bond 840).
Referring to FIG. 8C, for applications in which evanescent coupling
is used to guide light from the waveguide section to the bonded
die, patterning of the metal used for metal-assisted bonding of the
die is used to minimize optical loss at the points of greatest
field intensity in waveguide 830.
FIG. 9 illustrates an opto-electronic integrated circuit (OIEC)
according to an embodiment of the present invention. The OIEC 900
includes complex IC elements implemented by integrating active
devices with silicon photonic elements and silicon electronics
using composite integration. Referring to FIG. 9, a transceiver
with multiple optical transmit and receive channels is illustrated.
Multiple optical transmit channels 910 include hybrid integrated
tunable lasers and corresponding optical modulators as described in
relation to FIG. 7. Although MZMs are illustrated in FIG. 9, other
optical modulators are included within the scope of the present
invention. Additionally, although a heater is illustrated in
relation to the wavelength selective elements, other methods of
providing wavelength selectivity are included within the scope of
the present invention. Multiple optical receive channels 920, also
referred to as a set of optical receivers can include a waveguide
received bonded to the silicon photonic substrate as described in
relation to FIGS. 8A-8C. As illustrated in FIG. 9, complex digital
circuits capable of driving the high speed functions of the optical
sections, controlling the parameters of the optical sections,
performing electrical serialization and deserialization of the data
along with required framing for specific data protocols, and
high-speed electrical I/O are implemented on the silicon wafer, for
example, using CMOS processing. In other embodiments, the
transmitter section may include drivers capable of implementing
advanced modulation format optical signals and the receiver section
may support coherent detection. In such embodiments, circuit
elements such as wavelength trackers to control tunable lasers used
as local oscillators would be incorporated. Thus, embodiments
provide for implementation of advanced modulation format
transmitters and coherent receivers among other devices.
One or multiple channels of optical sources can be provided with
direct or external modulation. In the illustrated embodiment, each
optical transmitter includes a tunable laser integrated with an
optical modulator. One or multiple photosensitive regions may be
similarly provided for effecting receive functionality. A
transmitter drive section 912 is provided in communication with the
multiple optical transmit channels 910 to generate a signal for the
optical modulators and/or the optical sources used in the
transmitters. A receiver section 922 is provided for signal
amplification and to provide current to voltage conversion for the
multiple optical receive channels 920, also referred to as a set of
optical receivers.
Additionally, control electronics 930 may be provided to maintain
optical output levels within a desired range. A
serializer/deserializer (SERDES) section 940 is provided in the
illustrated embodiment with data framing capability to ensure data
is converted to or maintained in a specific data protocol.
Electrical input sections may be provided with clock and data
recovery or input equalization to effectively recover incoming
data. Electrical output drivers may be provided with output
pre-emphasis or other signal conditioning to drive output signals
over a prescribed distance of printed circuit board material. Such
drivers can be included in the CDRs, PLLs, and IO Drivers section
950.
It is also understood that the examples and embodiments described
herein are for illustrative purposes only and that various
modifications or changes in light thereof will be suggested to
persons skilled in the art and are to be included within the spirit
and purview of this application and scope of the appended
claims.
* * * * *
References